(1) Field of the Invention
The present invention relates to the field of producing permanent joints between various parts of articles, and more particularly to a method and an apparatus for fabricating polymetallic articles by diffusion bonding process.
This invention is especially well suited for application in aircraft industry, machine-building industry and pipe industry where polymetallic articles are extensively used.
In addition, the invention is readily adaptable for application where it is necessary to provide a reliable and sound bond between two or more heterogeneous metallic materials which have sufficiently high diffusion activity while undergoing concurrent heating.
At present, diffusion welding or solid-state diffusion bonding of metal parts, including those made of heterogeneous metals, finds extensive industrial use owing to the following advantages inherent therein, namely: the joints thus produced show high strength and occupy narrow areas at the surfaces being joined, the structure and properties of metal at the interfitting surfaces undergo but minimum change, the basic parameters of the process (temperature, pressure and time) being easy to control.
(2) Description of the Prior Art
Widely used in the machine-building industry is a method of diffusion bonding in vacuum, effected at normal temperature and providing a high-quality permanent joint between bonded parts of heterogeneous metals. However, this method is complicated to carry out because of the necessity to create high vacuum for the diffusion bonding process if, the metal parts to be joined are firmly pressed or clamped together under high vacuum.
Therefore, apparatus used for carrying out the aforedescribed diffusion bonding process are provided with a vacuum chamber equipped with intricate means for creating high vacuum, and since the metal parts to be joined require considerable force to hold them in sufficiently intimate contact to allow diffusion, this necessitates the use of presses or clamping arrangements to be mounted within said chamber. High clamping pressure is required due to the fact that the process of diffusion bonding is effected in vacuum at normal temperature when the diffusion activity of the atoms of the metals being joined is very low.
It follows that the arrangement of clamping means in the evacuated chamber poses a serious problem of constructional nature, especially where articles of large dimensions are dealt with.
There is known another method of diffusion bonding, wherein heat is used to activate diffusion bonding, said method being extensively employed in the production of tubes from bimetallic and polymetallic materials. This method ensures the provision of a high-quality joint or bond, but necessitates heating of two- or multi-layer metal parts in expensive furnaces with an inert or neutral atmosphere (necessary to protect the interfacing metal surfaces from oxidation). Likewise, it requires selection of materials appropriately arranged in layers so that the heat expansion coefficient thereof decreases in value from the inner to the outer layers, thereby providing for natural clamping of interfacing layers one to another.
There is also known a process for diffusion bonding (cf. U.S. Pat. No. 3,815,219, 1974) of an assembly of metal parts, said parts being both held in intimate contact and pressed together by mechanically applied pressure, which comprises preheating of said assembly in a suitable non-metallic, inorganic melt at atmospheric pressure, removing said assembly from said melt, pressing said parts together while hot and coated with the melt material, the coating of melt material protecting said assembly of metal parts from oxidation, reimmersing said assembly in said melt, and further heating it to a temperature at which interdiffusion between said metal parts takes place for a period of time sufficient to provide a satisfactory bond.
The metal parts to be diffusion bonded are joined and held in intimate contact, prior to being immersed in the melt, either by twisting thereof (in case of a wire), or by spot-welding of metal blocks clamped together by means of a mechanical or hydraulic press, or else by riveting said metal parts, the heat expansion coefficient of the rivet material being less than that of metals from which are fabricated the parts being bonded.
When the assembly of metal parts is removed from the high-temperature melt, it is subjected to rolling between working rolls to clamp the parts one to another (provided the assembly or article is simple in shape), or else said metal parts are subjected to a forge rolling operation.
The rolling or pressing of metal parts coated with the melt causes sticking of the latter to the working surfaces of such rolling or pressing units, thus necessitating the removal of the increasing layer of the melt material therefrom to avoid flaws or defects in the articles being worked on in such installations.
Moreover, the reduction or deformation of the metal parts coated with the film of said melt will invariably cause destruction of the film which protects the article and interfacing surfaces thereof from oxidation. The oxidation of said interfacing surfaces will either render the diffusion bonding process inoperative or considerably impair the quality of the bond due to the presence of oxidized films on the surface of the metal parts being joined together.
In order to eliminate the possibility of oxidation of the interfacing surfaces, it is necessary to provide the largest possible area whereupon interdiffusion between the preheated metal parts takes place. This, however, is possible only if the preheating cycle, and hence the entire operating process of diffusion bonding, is extensively long. In addition, special joining means are required for holding the metal parts together in intimate contact, for example, the aforedescribed means in the form of rivets fabricated from material whose heat expansion coefficient is lower than that of metals used for the parts being bonded.
The fact that the metal parts are exposed to the air and pressed or clamped to one another by means of a working tool having a temperature substantially lower than that of said parts, causes a sharp decrease in temperature of the assembly of metal parts and diminishes the deformation effect due to the interdependence between the diffusion activity and temperature. This relationship is governed by the parabolic law.
Therefore, said diffusion activity goes sharply down together with the falling temperature of said parts.
In view of inadequate workability of the metal parts in the air, said parts are reimmersed after deformation in the high-temperature melt for a long period of time to thereby strengthen the diffusion bonding therebetween, effected by preheating and subsequent mechanical treatment thereof.
Fluctuations in temperature of the assembly of metal parts, while being exposed to the air or subject to mechanical treatment, cause impairment of the metal structure in the bonding area, which becomes non-uniform in grain-size, especially in cases when one of the metal parts being bonded is made of austenite steel.
Where it becomes difficult to remove the assembly from the melt because of its thickness or to clamp the parts of said assembly together in the air, the clamping or pressing operation required for diffusion bonding is carried out by mechanical means directly in the melt at the high melt temperature.
However, the assembly operation according to the method described above is an exacting one since the assembly of metal parts and a clamping means should be disposed in a precise mutual relationship in the melt container which tends to be bulky because of this. In case this precise mutual arrangement is effected prior to immersing said assembly and clamping means in the melt, the pretreatment of the assembly of metal parts to be diffusion bonded is thus rendered difficult.
From the above, it follows that the prior-art method of diffusion bonding is of limited utility since it is incapable of providing diffusion bonding of complex-shaped, multilayer cylindrical articles, such as tubes, due to the impossibility of effecting mechanical clamping of metal parts of such an article, or because of the difficulty of operating said clamping means having working components of sophisticated designs.
Where the solid-state diffusion bonding process is used for fabricating articles made of metals showing high diffusion activity at the process temperature, it is sufficient to immerse the assembly of metal parts being bonded in the high-temperature melt without clamping said parts together either within the melt container or in the air.
As follows from the patent referred to above, the time period during which the immersed assembly remains in the melt container is 20 to 30 hours.
An apparatus for carrying the aforedescribed solid-state diffusion bonding method into effect is formed of a melt container arranged within a thermally insulated housing and heated by any convenient source of heat. Where it is indispensable to effect clamping of the metal parts within the melt container, any suitable clamping arrangement, such as a mechanical or hydraulic press, is mounted above the container to provide intimate contact between the interfaces of said metal parts.
When the clamping operation is effected during the removal of said parts from the melt container, a clamping arrangement (pressure rolls, press or forge machine) is mounted in close proximity with the container to bring the loss of temperature of the article to minimum.
The aforedescribed disadvantages of the prior-art diffusion bonding method narrow the scope of its application.
The primary object of the present invention is to provide a method and an apparatus for fabricating polymetallic articles by the diffusion bonding process, which will make it possible to reduce the time for pretreating an assembly of polymetallic parts being bonded.
Another object of the invention is to provide a method and an apparatus which will allow an increase in the variety range of articles being bonded.